The present invention relates to the recovery of production fluids from an oil or gas well having a christmas tree.
Christmas trees are well known in the art of oil and gas wells, and generally comprise an assembly of pipes, valves and fittings installed in a wellhead after completion of drilling and installation of the production tubing to control the flow of oil and gas from the well. Subsea Christmas trees typically have at least two bores one of which communicates with the production tubing (the production bore), and the other of which communicates with the annulus (the annulus bore). The annulus bore and production bore are typically side by side, but various different designs of Christmas tree have different configurations (ie concentric bores, side by side bores, and more than two bores etc).
Typical designs of christmas tree have a side outlet to the production bore closed by a production wing valve for removal of production fluids from the production bore. The top of the production bore and the top of the annulus bore are usually capped by a christmas tree cap which typically seals off the various bores in the christmas tree, and provides hydraulic channels for operation of the various valves in the christmas tree by means of intervention equipment, or remotely from an offshore installation.
In low pressure wells, it is generally desirable to boost the pressure of the production fluids flowing through the production bore, and this is typically done by installing a pump or similar apparatus after the production wing valve in a pipeline or similar leading from the side outlet of the christmas tree. However, installing such a pump in an active well is a difficult operation, for which production must cease for some time until the pipeline is cut, the pump installed, and the pipeline resealed and tested for integrity.
A further alternative is to pressure boost the production fluids by installing a pump from a rig, but this requires a well intervention from the rig, which can be even more expensive than breaking the subsea or seabed pipework.
According to the present invention there is provided a method of recovering production fluids from a well having a tree, the tree having a first flowpath and a second flowpath, the method comprising diverting fluids from a first portion of the first flowpath to the second flowpath, and diverting the fluids from the second flowpath back to a second portion of the first flowpath, and thereafter recovering fluids from the outlet of the first flowpath.
Preferably the first flowpath is a production bore, and the first portion of it is typically a lower part near to the wellhead. The second portion of the is first flowpath is typically an upper portion of the bore adjacent a branch outlet, although the second portion can be in the branch or outlet of the first flowpath.
The diversion of fluids from the first flowpath allows the treatment of the fluids (eg with chemicals) or pressure boosting for more efficient recovery before re-entry into the first flowpath.
Optionally the second flowpath is an annulus bore, or a conduit inserted into the first flowpath. Other types of bore may optionally be used for the second flowpath instead of an annulus bore.
Typically the flow diversion from the first flowpath to the second flowpath is achieved by a cap on the tree. Optionally, the cap contains a pump or treatment apparatus, but this can preferably be provided separately, or in another part of the apparatus, and in most embodiments, flow will be diverted via the cap to the pump etc and returned to the cap by way of tubing. A connection typically in the form of a conduit is typically provided to transfer fluids between the first and second flowpaths.
The invention also provides a flow diverter assembly for a tree, the flow diverter assembly comprising flow diverter means to divert fluids from a first portion of the first flowpath to a second flowpath, and means to divert fluids from the second flowpath back to a second portion of the first flowpath for recovery therefrom via the outlet of the first flowpath.
Typically, the diverter assembly can be formed from high grade steels or other metals, using eg resilient or inflatable sealing means as required.
The assembly may include outlets for the first and second flowpaths, for diversion of the fluids to a pump or treatment assembly.
The assembly preferably comprises a conduit capable of insertion into the first flowpath the assembly having sealing means capable of sealing the conduit against the wall of the production bore. The conduit may provide a flow diverter through its central bore which typically leads to a christmas tree cap and the pump mentioned previously. The seal effected between the conduit and the first flowpath prevents fluid from the first flowpath entering the annulus between the conduit and the production bore except as described hereinafter. After passing through a typical booster pump, squeeze or scale chemical treatment apparatus, the fluid is diverted into the second flowpath and from there to a crossover back to the first flowpath and first flowpath outlet.
The assembly and method are typically suited for subsea production wells in normal mode or during well testing, but can also be used in subsea water injection wells, land based oil production injection wells, and geothermal wells.
The pump can be powered by high pressure water or by electricity which can be supplied direct from a fixed or floating offshore installation, or from a tethered buoy arrangement, or by high pressure gas from a local source.
The cap preferably seals within christmas tree bores above the upper master valve. Seals between the cap and bores of the tree are optionally O-ring, inflatable, or preferably metal-to-metal seals. The cap can be retro-fitted very cost effectively with no disruption to existing pipework and minimal impact on control systems already in place.
The typical design of the flow diverters within the cap can vary with the design of tree, the number, size, and configuration of the diverter channels being matched with the production and annulus bores, and others as the case may be. This provides a way to isolate the pump from the production bore if needed, and also provides a bypass loop.
The cap is typically capable of retro-fitting to existing tree caps, and many include equivalent hydraulic fluid conduits for control of tree valves, and which match and co-operate with the conduits or other control elements of the tree to which the cap is being fitted.
In most preferred embodiments, the cap has outlets for production and annulus flow paths for diversion of fluids away from the cap.